A voltage-controlled oscillator (VCO) is an electrical oscillator having an output frequency controlled by a voltage input. FIG. 1 depicts a prior art VCO design employing an oscillator circuit 100 coupled to an LC tank 104 having a variable resonant frequency. A capacitance within the LC tank may be implemented as a capacitor bank 106, whose value may be selected by a nominal frequency control signal 108.
FIG. 2 shows an implementation of capacitor bank 106, wherein a plurality of selectable capacitances C.0 through C.(N−1) are coupled in parallel. In FIG. 2, the smallest capacitance C.0 has a value associated with a least significant bit (LSB) of the capacitor bank, while each successive capacitance C.n has nominally twice the value of the previous capacitance C.(n−1), i.e., the capacitances are binary-weighted.
In FIG. 2, each capacitance C.0 through C.(N−1) may be enabled or disabled by closing or opening, respectively, a corresponding switch S.0 through S.(N−1). By configuring the switches S.0 through S.(N−1), an arbitrary subset of the binary-weighted capacitances C.0 through C.(N−1) may be selected to vary the resonant frequency of the LC tank.
Ideally, the discrete step size of the capacitor bank, i.e., the change in actual capacitance from a selectable value of the capacitor bank to the next-largest selectable value, is uniformly 1 LSB over the entire range of selectable capacitances. In practice, however, the step size may deviate from 1 LSB due to mismatch, process variations, and/or other factors. FIG. 3 illustrates non-uniform step sizes typically encountered in practical implementations of a capacitor bank 106. From FIG. 3, it can be seen that the step size may deviate considerably over the range of selectable capacitances, causing significant error in the actual capacitance delivered by the capacitor bank. Note FIG. 3 is provided as an illustration only, and is not meant to restrict the scope of the present disclosure to any particular range of capacitance values or step sizes.
It would be desirable to have techniques to calibrate the capacitances available in a capacitor bank to provide more uniform step sizes and improved accuracy in selecting the oscillating frequency of a VCO.